The Fenton oxidation of biologically treated paper and pulp mill effluents: A performance and kinetic study

Accepted Manuscript Title: The Fenton oxidation of biologically treated paper and pulp mill effluents: A performance and kinetic study Authors: A. Brink, C.M Sheridan, K.G. Harding PII: DOI: Reference:

S0957-5820(17)30053-8 http://dx.doi.org/doi:10.1016/j.psep.2017.02.011 PSEP 978

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Process Safety and Environment Protection

Received date: Revised date: Accepted date:

29-10-2016 22-1-2017 12-2-2017

Please cite this article as: Brink, A., Sheridan, C.M, Harding, K.G., The Fenton oxidation of biologically treated paper and pulp mill effluents: A performance and kinetic study.Process Safety and Environment Protection http://dx.doi.org/10.1016/j.psep.2017.02.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

The Fenton oxidation of biologically treated paper and pulp mill effluents: A performance and kinetic study A. Brinka,b, C.M Sheridana,b*, K.G. Hardinga,b, a

Industrial and Mining Water Research Unit (IMWaRU), School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa

b

Centre in Water Research and Development (CiWaRD), University of the Witwatersrand, Johannesburg, South Africa [[email protected]]

Abstract The Fenton oxidation (Fe2+/H2O2) of bio-recalcitrant organics, which are present in biologically treated paper and pulp mill effluents (BTME), were investigated in this study. This study primarily focused on the performance and kinetics involved in the Fenton oxidation of BTMEs. A biologically treated recycle mill effluent (RME) and a neutral sulfite semi-chemical (NSSC) mill effluent were used for the experiments. The impact of FeSO4.7H2O and H2O2 dosages on chemical oxygen demand removal (COD) was evaluated. The Fenton oxidation experiments were carried out at 25°C and a pH value of 3.8. The initial COD of the biologically treated NSSC and RME effluents were 3756 mg/L and 436 mg/L, respectively. The maximum COD removal was found at a Fe2+/H2O2 ratio of 2.22 and 0.32 for the RME and NSSC effluents, respectively. The optimal COD/H2O2 for the RME and NSSC effluents was found to be 0.96 and 1.19 respectively. After a 60 minute reaction, the maximum COD removal efficiency for the NSSC and RME effluents were found to be 44% and 63%, respectively. The maximum reaction rates obtained for the RME and NSSC effluents were 18 mg COD.L-1.min-1 and 48 mg COD.L-1.min-1, respectively. The experimental results demonstrated that bio-recalcitrant organics, such as phenols and lignin, were readily degraded into organic acids. The applicability of the first order, second order, Behnajady– Modirshahla–Ghanbery (BMG) and a newly develped two staged first-order (TSF) kinetic model were evaluated. Both the BMG and TSF models yielded high correlation coefficients (r2). For extended reaction times, it was found that the TSF model best described the COD removal. In addition, the TSF kinetic constants (k12, k13) revealed that a rapid initial degradation reaction is followed by a slower secondary degradation reaction. This performance and kinetic study demonstrated that the conventional Fenton process can effectively remove bio-recalcitrant organics that are found in BTMEs. Keywords: Fenton process; Recycle mill effluent; Neutral sulfite semi chemical mill effluent; chemical oxygen demand

1. Introduction One of the more pressing environmental concerns related to the paper and pulp industry is the production of organic rich wastewaters. Paper and pulp mill effluents generally contain cellulosic material, lignin, phenols, chlorinated and sulfite complexes (Carg, 2012). The biodegradable matter present in these mill effluents are generally removed by anaerobic and aerobic digestion processes (Kamali and Khodaparast, 2015; Meyer and Edwards, 2014). However, certain mill g still contain a large fraction of bio-recalcitrant aromatic/phenols organics such as lignin and humic acids (Blank et al., 2016; Lindholm-Lehto et al., 2015). The direct discharge of biologically treated mill effluents (BTME) can therefore still have a significant impact on surrounding waterbodies. The toxicity of BTMEs are primarily caused by lignin derivatives such as phenols, resins, lignosulphonic acids and other hydrocarbons (Garg and Tripathi, 2011; Raj et al., 2007). The phenols present in BTMEs are extremely toxic to aquatic ecosystems, even at low concentrations (Alver et al., 2015). Certain alkyl phenols originating from the paper and pulp industry can be considered toxic to the aquatic environment at concentrations as low as 1- 20 mg/L (Staples et al., 2002). In addition, resin acids can accumulate in sediment and are responsible for chronic and acute toxicity in fish species (Liss et al., 1997). The dark colour of certain BTMEs lowers the aesthetic water quality of waters and hinders natural photosynthesis in aquatic systems (Murugesan, 2003; Kannan and Oblisami, 1990). Consequently, the treatment of bio-recalcitrant organics is considered to be an important aspect for future environmental preservation. Recently, advanced oxidation processes (AOP), such as the Fenton process, yielded promising results for the treatment of recalcitrant organics that are present in paper and pulp mill effluents (Garcia-Segura et al., 2016;Kamali and Khodaparast, 2015; Rabelo et al., 2014; Zahrim et al., 2007). In AOP treatment systems, organic constituents are mineralized and eliminated rather than transferred, concentrated and collected as in other tertiary treatment systems (Pouran et al., 2015; Sharma et al. 2011). The Fenton process have shown to be an effective treatment technology to remove refractory phenols and lignin from Kraft mill effluents (Arantes and Milagres, 2007). This AOP technology is favoured since it is associated with high organic removal rates, as well as low capital and operational costs (Alver, et al., 2015). The research for industrial scale applications, such as fluidized-bed Fenton (FBF) columns (Garcia-Segura et al., 2016), is still limited and more bench scale studies are required before Fenton technologies can be scaled up (Tambosi et al. 2006; Ginni et al. 2014). The efficiency of Fenton related processes is generally based upon the catalytic formation and equilibrium concentration of hydroxyl radicals (OH*) (Babuponnusami and

Muthukumar, 2014; Maezono et al., 2011). The conventional Fenton process is primarily described by reaction (1) - (7) (Wu et al., 2010):

𝐹𝑒 2+ + 𝐻2 𝑂2 → 𝐹𝑒 3+ + 𝑂𝐻 ∗ + 𝑂𝐻 −

(1)

𝑂𝑟𝑔𝑎𝑛𝑖𝑐𝑠 + 𝑂𝐻 ∗ → 𝐻2 𝑂 + 𝐶𝑂2 + 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠

(2)

𝑂𝐻 ∗ + 𝐻2 𝑂2 → 𝐻2 𝑂 + 𝐻𝑂2∗

(3)

𝐻2 𝑂2 + 𝐹𝑒 3+ → 𝐹𝑒 2+ + 𝐻𝑂2∗ + 𝐻 +

(4)

𝐹𝑒 3+ + 𝐻𝑂2∗ → 𝐹𝑒 2+ + 𝑂2 + 𝐻 +

(5)

𝐹𝑒 2+ + 𝐻𝑂2∗ → 𝐹𝑒 3+ + 𝑂𝐻 −

(6)

𝑂𝐻 ∗ + 𝐹𝑒 2+ → 𝑂𝐻 − + 𝐹𝑒 3+

(7)

Various studies have investigated the removal of organics from raw paper and pulp mill effluents using Fenton related oxidation processes (Perez et al., 2002; Torrades et al., 2003; Rabelo et al., 2014; Zahrim et al., 2007). These Fenton related oxidation processes include the conventional Fenton process (Gholami et al., 2016), photo-Fenton process (Rabelo et al., 2014), solar photoFenton process (Fernandes et al., 2014), electro-Fenton process (Un et al., 2015) and Fentonlike process (Eskelinen et al., 2010). However, only a few studies evaluated the performance of Fenton oxidation processes treating biologically treated paper and pulp mill effluents (Catalkaya and Kargi, 2007; Ginni et al., 2014). There is also a shortage of studies investigating the mechanisms and kinetics involved in the homogenous Fenton oxidation of BTME (Wang et al., 2011). As a result, the aim of this study was to investigate the performance, degradation pathways and kinetics involved in the Fenton oxidation of BTMEs. The primary objectives of this study are to assess (i) the influence of Fe2+ and H2O2 dosages on organic removal efficiencies and (ii) to investigate the applicability of several kinetic models.

2. Material and Methods 2.1.

Fenton reagents and chemical analysis

The FeSO4.7H2O (Merck) (Code: 103965) and H2O2 (Merck, 30% w/v) (Code: 107209) were used as the Fenton’s reagent. The pH of the solution was altered using analytical grade H2SO4 (Merck) (Code: 100731). MnO2 (Merck) (Code: 105957) was immediately added to the samples taken directly from the original reaction batch to remove excess H2O2 prior to sample analysis. The absence of residual H2O2 was confirmed using an MQaunt test strips (0.5 – 25 mg/L H2O2) (Code: 110011). The COD, volatile organic acids (VOA) and phenolic measurements were determined with calorific methods and measured on a Spectroquant®. A Merck COD cell test (100-1500 mg/L) (Code: 114539), volatile organic acid cell test (50-3000 mg/L) (Code: 101809) and phenol test (0.002-5 mg/L) (Code: 100856) were used to characterise the effluent. The equivalent lignin content was measured using UV-methods. A calibration curve was obtained at 267 nm using Kraft lignin (Sigma-Aldrich) (Code: 370959) (Wang et al., 2014). The pH values of the samples were measured using a handheld IP67 Combo pH/COND/D.O. (8603) meter. The total suspended solids (TSS) were measured according to standard methods (Skrentner, 1988). 2.2.

Experimental procedures

The untreated recycle (RME) and neutral sulfite semi chemical (NSSC) mill effluents were initially treated using an aerobic moving bed biofilm reactor (MBBR). The MBBR was operated at a hydraulic residence time (HRT) of 24 hours. After biological treatment, the samples were stored at 4°C. In the experiments the effect of FeSO4.7H2O and H2O2 were investigated. In the first set of experiments, H2O2 dosages of 450 mg/L, 3150 mg/L and 6300 mg/L was used for both the RME and NSSC effluents. The generally accepted theoretical H2O2/COD ratio is 2.125 g H2O2/g COD to fully oxidize organics. However, optimal reaction rates are often found at H2O2/COD ratios significantly lower than this theoretical ratio due to side reactions occurring (Sevimli et al., 2014; Ertugay and Acar, 2013; Barbusi and Pieczykolan, 2010). Consequently, H2O2/COD ratios lower than the theoretical amount was also evaluated in this study. In the experimental work the initial H2O2/COD ratios for the RME effluent ranged between 1.03 (g/g) and 14.45 (g/g). For the NSSC effluent, the H2O2/COD ratios were varied between 0.12 (g/g) and 1.68 (g/g). For each individual H2O2 dosage three different FeSO4.7H2O dosages were evaluated. The corresponding FeSO4.7H2O dosages were 50 mg/L, 500 mg/L and 1000 mg/L. The initial pH value of the solution was adjusted to 3.8 using H2SO4. All the experiments were carried out in a 250 mL Erlen Meyer

flask at room temperature (25°C) and constant agitation (3000 rpm). The reaction time for all these experiments was 60 minutes.

In a separate set of experiments, the reaction time was increased from 60 to 240 minutes for the NSSC effluents to evaluate the impact of extended reaction times on COD removal. The H2O2 dosages were 3150 mg/L and 6300 mg/L, respectively. The corresponding FeSO4.7H2O dosages for each individual H2O2 dosage were 500 mg/L and 1000 mg/L, respectively.

2.4.

Kinetic models

2.4.1. First order kinetic model The rate of organic removal (COD) in terms of first order rate laws can be described by the following expression (Khamaruddin et al., 2011; Wang, 2008): 𝑑𝐶𝐶𝑂𝐷 = −𝑟𝐶𝑂𝐷 = −𝑘2 𝐶𝐶𝑂𝐷 . 𝐶𝑂𝐻 ∗ 𝑑𝑡

(8)

= 𝑘𝑎𝑝𝑝,1 . 𝐶𝐶𝑂𝐷 According to Wu et al. (2010), hydroxyl radicals (OH*) only have a lifetime of a few nanoseconds and is present in low concentrations. Hence, it can be assumed that the concentration of the hydroxyl radicals (𝐶𝑂𝐻∗ ) is constant. When Eq. (8) is integrated, the following equation is derived:

ln (

𝐶𝐶𝑂𝐷

𝐶𝐶𝑂𝐷𝑜

) = −𝑘𝑎𝑝𝑝,1 . 𝑡

(9)

Where 𝐶𝐶𝑂𝐷𝑜 and 𝐶𝐶𝑂𝐷 is the initial and effluent COD concentrations (mg/L), 𝑘𝑎𝑝𝑝,1 the apparent first order rate constant (min-1) and 𝑡 the reaction time (min). To obtain the apparent first order kinetic parameter (𝑘𝑎𝑝𝑝,1 ), 𝑙𝑛(𝐶𝐶𝑂𝐷 /𝐶𝐶𝑂𝐷𝑜 ) was plotted against the time (𝑡). The gradient of this line represents the apparent first order kinetic parameter (𝑘𝑎𝑝𝑝,1 ).

2.4.2. Second order kinetic model The rate of substrate removal (𝑟𝐶𝑂𝐷 ) described by second order reaction kinetics can be given by Eq. (10) (Guedes et al., 2003):

𝑑𝐶𝐶𝑂𝐷 2 = −𝑟𝐶𝑂𝐷 = −𝑘𝑎𝑝𝑝 ,2 . 𝐶𝐶𝑂𝐷 𝑑𝑡

(10)

Eq. (10) can be simplified by means of integration to yield the following linear expression: 1 𝐶𝐶𝑂𝐷𝑜

−

1 𝐶𝐶𝑂𝐷

= −𝑘𝑎𝑝𝑝,2 . 𝑡

(11)

Where 𝐶𝐶𝑂𝐷𝑜 and 𝐶𝐶𝑂𝐷 represents the initial and effluent COD concentration (mg.L-1), 𝑘𝑎𝑝𝑝,2 the second order rate constant (L.mg-1.min-1). The second order kinetic parameter (𝑘𝑎𝑝𝑝,2 ) is obtained by plotting the (1/𝐶𝐶𝑂𝐷𝑜 − 1/𝐶𝐶𝑂𝐷 ) value against the time (𝑡). The slope of Eq. (11) will represent the second order kinetic parameter (𝑘𝑎𝑝𝑝,2 ).

2.4.3. Behnajady–Modirshahla–Ghanbery (BMG) model A mathematical model was developed by Behnajady et al. (2007) to describe the rate of substrate removal during Fenton oxidation. This model is given by the following expression: 𝐶𝐶𝑂𝐷 𝑡 =1− 𝐶𝐶𝑂𝐷𝑜 𝑚 + 𝑏𝑡

(12)

Where 𝐶𝐶𝑂𝐷𝑜 and 𝐶𝐶𝑂𝐷 represents the initial and effluent COD concentrations (mg/L), 𝑡 the reaction time (min), 𝑚 (min) and 𝑏 are the kinetic parameters. These values can theoretically be determined by taking the derivative of Eq. (12), which yields the following expression: 𝐶 𝑑 ( 𝐶𝑂𝐷 ) 𝐶𝐶𝑂𝐷𝑜 𝑚 =− (𝑚 + 𝑏𝑡)2 𝑑𝑡

(13)

When the time (𝑡) is short or approaching zero, Eq. (13) can be manipulated to give the following equation: 𝐶 𝑑 ( 𝐶𝑂𝐷 ) 𝐶𝐶𝑂𝐷𝑜 1 =− 𝑑𝑡 𝑚

(14)

The 1/𝑏 value represents the maximum substrate removal and can be calculated by the following expression: 𝐶𝐶𝑂𝐷 ∞ 1 =1− 𝑏 𝐶𝐶𝑂𝐷 𝑜

(15)

2.4.4. Two staged first order kinetic (TSF) model A study conducted by Lei and Li (2014), demonstrated that the COD removal during the ozonation of Kraft mill effluent exhibited a two staged first order behaviour. Unfortunately, the two staged first order (TSF) kinetic model was not developed and evaluated in Lei and Li (2014). As a result, this study develops and assesses the applicability of the TSF model. In the TSF model, it was assumed that the chemical oxygen demand (COD) can be divided into a rapid degradable (CODrd) and a slowly degradable (CODsd) fraction. The rapidly degradable COD (CODrd) are assumed to be aromatic constituents such as lignin and phenols, while the slowly degradable COD (CODsd) are generally carboxylic acids (Lopez et al., 2004). Literature have shown that the Fenton oxidation of lignin, phenols and carboxylic acids follow first order rate laws (Zazo et al., 2005; Passauer et al., 2011; Makhotkina et al., 2008). Subsequently, it is expected that the kinetics describing COD removal will reflect this same first order behaviour. The model further assumes that organic type A contributes to the readily degradable COD (CODrd), while organic type B primarily contributes to the slowly degradable COD (CODsd). The oxidising reactions for both organic type A and B are presented in Eqs. (16) and (17). 𝑘12

(16)

𝑘13

(17)

𝐶𝐴 + 𝑂𝐻∗ → 𝐶𝐵

𝐶𝐵 + 𝑂𝐻 ∗ → 𝐶𝑂2 + 𝐻2 𝑂

The differential equations describing the removal of type A and type B organics are presented in Eqs. (18) and (19). 𝑑𝐶𝐴 = −𝑘12 . 𝐶𝐴 𝑑𝑡

(18)

𝑑𝐶𝐵 = 𝑘12 . 𝐶𝐴𝑂 . e−𝑘12 .𝑡 − 𝑘13 . 𝐶𝐵 𝑑𝑡

(19)

The analytical solution for 𝐶𝐴 and 𝐶𝐵 is given by Eqs. (20) and (21), respectively. 𝐶𝐴 = 𝐶𝐴𝑂 . e−k12 .𝑡

(20)

𝑒 𝑡.(𝑘13 −𝑘12 ) 1 ]] 𝐶𝐵 = ( 𝑘 .𝑡 ) [𝐶𝐵𝑂 + (𝑘12 . 𝐶𝐴𝑂 ) [ − 13 𝑒 𝑘13 − 𝑘12 𝑘13 − 𝑘12

(21)

1

Where 𝐶𝐴𝑂 and 𝐶𝐵𝑂 represent the initial concentrations of organic type A and B (mg/L), 𝑡 the reaction time (min), 𝑘12 and 𝑘13 the first order rate constants (min-1). The individual COD values of organic A and B can be calculated using Eqs. (22) and (23).

𝐶𝐶𝑂𝐷𝑟𝑑 = 𝑥. 𝐶𝐴

(22)

𝐶𝐶𝑂𝐷𝑠𝑑 = 𝑦. 𝐶𝐵

(23)

Where 𝑥 and 𝑦 represent the COD conversion constants (mg COD/mg A or B), 𝐶𝐶𝑂𝐷𝑠𝑑 and 𝐶𝐶𝑂𝐷𝑟𝑑 the slowly and rapidly degradable COD concentrations (mg/L), respectively. The total COD concentration can be given as a function of the rapidly (A) and slowly (B) degradable organics as seen in Eqs. (24) and (25). 𝐶𝐶𝑂𝐷 𝑜 = 𝐶𝐶𝑂𝐷𝑠𝑑 ,𝑜 + 𝐶𝐶𝑂𝐷𝑟𝑑 ,𝑜 = 𝑥. 𝐶𝐴𝑂 + 𝑦. 𝐶𝐵𝑂

(24)

𝐶𝐶𝑂𝐷 = 𝐶𝐶𝑂𝐷𝑠𝑑 + 𝐶𝐶𝑂𝐷𝑟𝑑 = 𝑥. 𝐶𝐴 + 𝑦. 𝐶𝐵

(25)

If the analytical solutions for 𝐶𝐴 and 𝐶𝐵 are substituted into Eq. (25), the COD concentration can be given by Eq. (26) which is a function of time (t) and the initial concentrations of organic type A and B.

−k12 .𝑡

𝐶𝐶𝑂𝐷 = 𝑥. 𝐶𝐴𝑂 . e

𝑒 𝑡.(𝑘13 −𝑘12 ) 1 ]] + ( 𝑘 .𝑡 ) [𝐶𝐵𝑂 + (𝑘12 . 𝐶𝐴𝑂 ) [ − 13 𝑒 𝑘13 − 𝑘12 𝑘13 − 𝑘12 𝑦

(26)

By substituting 𝐶𝐴𝑂 from Eq.(24) into Eq. (26), the following equation can be obtained which represents the TSF model.

𝐶𝐶𝑂𝐷 = (𝐶𝑂𝐷𝑜 − 𝑦. 𝐶𝐵𝑂 ). 𝑒 −𝑘12.𝑡 + (

𝑦 𝑒 𝑘13.𝑡

) [𝐶𝐵𝑂 + (

𝑘12 (𝐶𝑂𝐷𝑜 − 𝑦. 𝐶𝐵𝑂 ) [𝑒 𝑡.(𝑘13−𝑘12) − 1]] 𝑥(𝑘13 − 𝑘12 )

(27)

The initial concentration of the slowly degradable constituents was assumed to be negligible (𝐶𝐵𝑂 ≈ 0). The 𝑦/𝑥 ratio, 𝑘12 and 𝑘13 parameters were obtained by fitting the experimental data to the TSF model given by Eq. (27). The equations were solved using the solver function in Microsoft Excel.

3. Results and Discussion 3.1.

Paper and pulp mill effluent characterisation

Recycle (RME) and neutral sulfite semi-chemical (NSSC) mill effluents were collected from separate mills. After collection, the samples were stored at 4°C. The samples were treated in a lab scale aerobic moving bed biofilm reactor (MBBR) to resemble biologically treated mill effluents. The MBBR removed 32% and 55% of the COD from the untreated NSSC and RME effluent at a hydraulic residence time (HRT) of 24 hours. The characteristics of the biologically treated paper and pulp mill effluents are shown in Table 1.

3.2.

Effect of H2O2 and FeSO4.7H2O dosages on COD removal efficiency

The effect of H2O2 and FeSO4.7H2O dosages on the COD removal is evaluated in this section. The optimal pH was found to be between 3 and 4, which corresponds with values found in literature (Badawy et al., 2006; Sevimli et al., 2014). Consequently, an initial pH value of 3.8 was used for the experiments. The COD removal results for the Fenton oxidation of NSSC and RME effluents are illustrated in Fig. 1 and Fig. 2 respectively. The optimum COD removal for the NSSC effluent was found at a FeSO4.7H2O and H2O2 dosage of 1000 mg/L and 3150 mg/L, respectively. For the RME effluent, the optimum COD removal was found to be at a FeSO4.7H2O and H2O2 dosage of 1000 mg/L and 450 mg/L, respectively. At optimum conditions, the phenol degradation

was greater than 85% for both effluents. The experimental results suggest that an increase in catalyst (Fe2+) dosages generally leads to increase COD removal efficiency. The increase in catalyst (Fe2+) concentrations coincides with the increase in the formation of active hydroxyl radicals (OH*), as seen in Eq. (28). Higher oxidation rates of organic constituents (R-H) will consequently be the direct result of higher hydroxyl radicals (OH*) concentrations. 𝐹𝑒 2+ + 𝐻2 𝑂2 → 𝑂𝐻 ∗ + 𝑂𝐻 − + 𝐹𝑒 3+

(28)

𝑅𝐻 + 𝑂𝐻∗ → 𝑅∗ + 𝐻2 𝑂

(29)

However, excessive catalyst (Fe2+) dosages can decrease COD removal efficiency as seen in Fig. 1 a. According to Sevimli et al. (2014), excessive Fe2+ catalyst can react with the hydroxyl radicals (OH*), which evidently decreases the COD removal efficiency as seen in Eq. (30). 𝐹𝑒 2+ + 𝑂𝐻∗ → 𝐹𝑒 3+ + 𝑂𝐻−

(30)

The increase in hydrogen peroxide (H2O2) concentrations can also have both positive and negative implications on the COD removal efficiency. An increase in hydrogen peroxide (H2O2) can lead to an increase in COD removal efficiency due to the additional formation of hydroxyl radicals (OH*) as seen in Eq. (28). However, excessive hydrogen peroxide concentrations can have negative implications on the COD removal efficiency as seen in Fig. 1 and Fig. 2. This rather unique behaviour can be explained by the fact that hydrogen peroxide (H2O2) in excess can react with the active hydroxyl radicals to form water and perhydroxyl radicals (OH2*). The scavenging of active hydroxyl radicals (OH*) is shown in Eq. (31). 𝐻2 𝑂2 + 𝑂𝐻 ∗ → 𝐻𝑂2∗ + 𝐻2 𝑂

(31)

Consequently, the COD removal efficiency will drop due to the scavenging of hydroxyl radicals (OH*). The most efficient and economical solutions for the Fenton oxidation of wastewaters can be determined using the Fe2+/H2O2 and COD/H2O2 ratios. The ratios vary greatly in literature, likely due to the dependency on the type of contaminant treated in the Fenton process. In a study done by El Haddad et al. (2014) on the Fenton treatment of azo dyes, the optimal Fe2+/H2O2 ratio was found to be 0.1. In another study conducted by Ponuwei (2009) on the treatment of paper and pulp mill effluents using Fenton oxidation processes, the optimal ratio Fe 2+/H2O2 ratio was measured as 0.82. This study showed that the optimal Fe 2+/H2O2 ratio for the NSSC and RME

effluents were 0.32 and 2.22, respectively. In a study conducted on white liquor by Sevimli et al. (2014), an optimal COD removal was found at a COD/H2O2 ratio of 0.865. Similar results was obtained by Jarpa et al. (2016), where a maximum amount of COD was removed from Kraft mill effluent at a COD/H2O2 ratio of 1.12. In this study, the optimal COD/H2O2 ratio for the RME and NSSC effluents was found to be 0.96 and 1.19 respectively.

Due to the complexity of Fenton reactions, polynomial multiple regression models are frequently used to predict the removal of contaminants (Mojtaba and Soghraa, 2014; Lak et al., 2012). In this study, a reduced cubic polynomial model was used to graphically illustrate the impact FeSO4 and H2O2 dosages. The graphical illustrations depicting the COD removal efficiencies (%) are shown in Fig. 3 and Fig. 4 for the NSSC and RME effluents, respectively. The graphical illustrations were generated using Stat-Ease Design Expert ®.

The maximum COD removal efficiency can be predicted for a specific FeSO 4.7H2O and H2O2 dosage which can be calculated according to Eq. (32), where x1 and x2 represent the FeSO4.7H2O and H2O2 dosages, respectively. The predicted values represent the COD removal efficiencies after a 60 minute reaction period. The constant values (Ai, Bi, Ci, Di, Ei, Fi, Gi, Hi) for the RME and NSSC effluents are listed in Table 2. 𝐶𝑂𝐷𝑅 (%) = 𝐴𝑖 + 𝐵𝑖 . 𝑥1 + 𝐶𝑖 . 𝑥2 + 𝐷𝑖 . 𝑥1 . 𝑥2 + 𝐸𝑖 . 𝑥12 + 𝐹𝑖 . 𝑥22 + 𝐺𝑖 . 𝑥12 . 𝑥2 + 𝐻𝑖 . 𝑥1 . 𝑥22

(32)

These models are applicable for a H2O2 dosage between 450 mg/L - 6300 mg/L and a FeSO4.7H2O dosage between 50 mg/L - 1000 mg/L. The absolute values for the Bi constant were found to be significantly higher than that of the Ci values, for both effluents. Consequently, the amount of catalyst (Fe2+) has a greater effect on the overall COD removal than the amount of H2O2. Other studies have also demonstrated that the impact of catalyst (Fe2+) dosage have a more pronounced effect on contaminant removal in the Fenton process (Molina et al., 2006; Tony and Bedri, 2014).

3.3.

Kinetic model evaluation

The mechanisms and kinetic models behind the Fenton oxidation of paper and pulp mill effluent are discussed in this section. The first order, second order and BMG model are well-known models used to describe the removal of organics in the Fenton process (Tunc et al., 2012; Cui et al., 2014). The kinetic parameters and corresponding correlation coefficients (r2) for these models are presented in Table 3 and Table 4. The maximum reaction rates found for the RME and NSSC effluents were 18 mg COD. L-1. min-1 and 48 mg COD. L-1. min-1, respectively. The high correlations (r2) found for the BMG model would suggest that this model is best suited to describe the COD removal rate during the Fenton oxidation of biologically treated paper and pulp mill effluents.

The lower correlation coefficient (r2) values found for the first and second order plots can be explained by a two staged degradation of paper and pulp mill effluents. The first order kinetic plot in Fig. 5 illustrates the typical two staged degradation behaviour. The correlation coefficient for the complete 240 minute Fenton oxidation reaction might be relatively low (r 2 = 0.44), however the correlation coefficient for the α (r2 = 0.98) and β (r2 = 0.99) regions are high. This suggests that two separate first order degradation pathways may be responsible for the COD removal. The α and β regions represents rapid and slow first order reactions, respectively.

Similar findings were reported in Lei and Li (2014), where the ozonation of Kraft mill effluent illustrated that a two staged first-order model could potentially be used to describe the COD removal. Another study conducted by Wang (2008) on the Fenton oxidation of azo dyes illustrated that the degradation pathways followed combined first order kinetics, which were dependent on two different initial dye concentrations.

According to Oturan et al. (2008), the degradation of aromatic constituents during Fenton oxidation resulted in the formation of slowly degradable short chained carboxylic acids. Subsequently, the first rapid reaction step is mainly responsible for the reduction of aromatic molecules (α), followed by the slower degradation of carboxylic acids (β). The aromatic constituents in paper and pulp mill effluents are generally lignin and phenolic derived compounds. The initial step in lignin and phenol oxidation include the hydroxylation of the aromatic ring,

followed by the formation of mono and dicarboxylic acids (Zazo et al., 2005; Passauer et al., 2011). Intermediate carboxylic acids can include muconic acid, maleic acid, fumaric acid, oxalic acid, acetic acid and formic acid (Zazo et al., 2005). A separate Fenton oxidation experiment was conducted to confirm the degradation pathways of aromatic constituents present in the NSSC mill effluents. The change in phenols, lignin and acids (measured as pH) concentrations are illustrated in Fig. 6. The drop in pH during the course of the reaction could indicate that lignin and phenols are converted into intermediate organic acids. The lignin and phenols removal efficiency was found to be 78% and 87%, respectively. The results from Fig. 6 indicate that lignin and phenols are potentially converted into organic acids, since a pH drop is noticed. Other studies have also attributed the drop in pH to the formation of intermediate organic acids during the fractionation of aromatic and aliphatic constituents (Basu et al., 1997).

The kinetic parameters for the TSF model, first order kinetic model and BMG model are presented in Table 5. The kinetic parameters for the first order and BMG model differ from that of Table 3, due to extended reaction times (240 minutes). The TSF model takes the transformations of lignin and phenols into intermediate organic acids into account. As a result, the TSF model was only evaluated on the biologically treated NSSC effluent due to the high aromatic content (1840 mg/L lignin, 90 mg/L phenols) of the effluent. The reaction times were extended from 60 to 240 minutes to demonstrate that a slower secondary reaction is more dominant at final stages of the reaction. The kinetic results in Table 5, indicate that the TSF model had the highest correlation coefficients (r2). As a result, the TSF model would be best suited to describe the COD removal rate during the extended Fenton oxidation of bio-treated mill effluents.

The kinetic parameters (k12, k13) for the TSF model indicated that the initial degradation reaction is fast which is simultaneously followed by a slower reaction. The phenol, lignin and volatile organic acid (VOA) content of the BTME was also measured during the experiments to determine the corresponding first order rate constants. These first order rate constants were compared with the theoretical constants (k12, k13) obtained from the TSF model. According to Babuponnusami and Muthukumar (2012), the first order constant (k) for phenol degradation by means of Fenton oxidation are 0.0067 min-1 at Fe2+ and H2O2 dosages of 4 mg/L and 800 mg/L, respectively. Other Fenton related treatment systems yielded a first order rate constant (k) of 0.0934 min-1 for phenol

degradation. In Wang et al. (2014), the first order rate constant for lignin degradation via a Fenton process was 0.0498 min-1. In this study the first order rate constant for lignin and phenol degradation was found to be 0.0349 min-1 and 0.0652 min-1 respectively at a dosage of 500 mg/L FeSO4 and 3150 mg/L H2O2. The lignin and phenol first order rate constant found in the experiments are comparable with k12 found in Table 5 for the same dosage conditions. Experimental data from Centi et al. (2000) illustrated that the combination of zeolite and Fe2+ type catalysts for the Fenton oxidation of intermediate organic acids, such as formic acid and acetic acid, yielded first order constants of 0.0072 min-1 and 0.0018 min-1 respectively. At a dosage of 500 mg/L FeSO4 and 3150 mg/L H2O2, the change in volatile organic acids (VOA) yielded a first order rate constant (k) of 0.0011 min-1, which is comparable with 𝑘13 in Table 5 for the same dosage conditions. At a dosage condition of 500 mg/L FeSO4 and 3150 mg/L H2O2, the y/x ratio was found to be 0.79 as seen in Table 5. The y/x ratio found can possibly indicate the type of organic transformation taking place during the Fenton oxidation process. The hydroxylation of lignin and phenolic structures generally yields y/x ratios close to 0.79. The breakdown of aromatic derived muconic acid into acetic acid will also result in y/x ratios close to 0.79. The change in total COD, readily CODrd and slowly degradable CODsd can be seen in Fig. 7.

4. Conclusion This study investigated the treatment performance, degradation pathways and kinetics involved in the Fenton oxidation of biologically treated mill effluents (BTME). One of the primary objectives was to evaluate the impact of Fe2+ and H2O2 dosages on the COD removal efficiency. It was found that the optimal Fe2+/H2O2 and COD/H2O2 ratio for the NSSC effluent was 0.32 and 1.19, respectively. Whereas the optimal Fe2+/H2O2 and COD/H2O2 ratios for the RME effluent were found to be 2.22 and 0.96, respectively. The constants in the reduced cubic polynomial models revealed that the influence of catalyst dosage (Fe2+) appears to have a more pronounced effect on COD removal efficiencies. After a 60 minute reaction time, the maximum COD removal efficiency for the NSSC and RME effluents were found to be 44% and 63%, respectively. The second objective of this study was to assess the applicability of various kinetic models to describe the organic removal rates. The first order, second order, BMG and newly developed TSF model were evaluated. Both the BMG and TSF model generally had high correlation coefficients (r2). However, during extended Fenton oxidation reactions the TSF model yielded the highest

correlation coefficients (r2=0.99). The TSF kinetic model constants k12 and k13 were found to be comparable to the first order kinetic constants that described the degradation of aromatics and of carboxylic acids, respectively. The kinetic results for the TSF model revealed that the k12 value is significantly higher than the k13 value. Hence, rapidly degradable aromatic constituents such as lignin and phenols are converted into slowly degradable carboxylic acids. The readily biodegradable organic acids formed during the Fenton oxidation of PPMEs can be removed with tertiary biological systems in future studies. The performance and kinetic results of this study demonstrates that the Fenton process can effectively treat bio-recalcitrant organics present in BTMEs.

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Fig. 1 – The effect of various H2O2 and FeSO4.7H2O dosages on the COD removal efficiency for the NSSC effluent at pH=3.81 (▲ = 50 mg/L FeSO4.7H2O; ● = 500 mg/L FeSO4.7H2O; ■ = 1000 mg/L FeSO4.7H2O)

Fig. 2 – The effect of various H2O2 and FeSO4.7H2O dosages on the COD removal efficiency for the RME effluent at a pH=3.8 (▲ = 50 mg/L FeSO4.7H2O; ● = 500 mg/L FeSO4.7H2O; ■ = 1000 mg/L FeSO4.7H2O)

Fig. 3 – The COD removal efficiency (%) at various H2O2 and FeSO4.7H2O dosages (NSSC effluent; 60 minute reaction time)

Fig. 4 – The COD removal (%) at various H2O2 and FeSO4.7H2O dosages (RME effluent; 60 minute reaction time)

Fig. 5 – First order kinetic plot for the NSSC effluent (FeSO4.7H2O=500 mg/L; H2O2=3150 mg/L)

Fig. 6 – The change in lignin (L;Lo) ●, phenol (P;Po) ▲ and pH ■ during Fenton oxidation (FeSO4.7H2O = 500 mg/L; H2O2 = 3150 mg/L)

Fig. 7 – The TSF model illustrating the change in the overall COD, CODrd and CODsd for the NSSC effluent with a FeSO4.7H2O and H2O2 dosage of 500 mg/L and 3150 mg/L (● experimental COD values)

Table 1: The wastewater characteristics of the biologically treated RME and NSSC effluents Parameters

Recycle mill effluent (RME)

Neutral sulfite semi chemical effluent (NSSC)

COD (mg/L)

436 ± 11.00

3756 ± 92.00

VOA (mg/L)

221 ± 6.00

951 ± 26.00

Phenols (mg/L)

6.9 ± 0.12

90 ± 1.59

Lignin (mg/L)

35 ± 7.00

1840 ± 92.15

TSS (mg/L)

45 ± 2.25

278 ± 13.90

7.67 ± 0.10

8.09 ± 0.10

1374 ± 13.47

4060 ± 40.60

pH Conductivity (µS/cm)

Table 2: Individual factorial design parameters for the NSSC and RME effluents Polynomial parameters

NSSC effluent

RME effluent

𝐴𝑖

15.10

20.04

𝐵𝑖

-0.02

0.12

𝐶𝑖

-8.82E-04

5.32E-03

𝐷𝑖

1.78E-05

-2.50E-05

𝐸𝑖

9.80E-06

-7.12E-05

𝐹𝑖

1.83E-07

7.77E-07

𝐺𝑖

-

7.78E-09

𝐻𝑖

-2.05E-09

1.97E-09

Table 3: The kinetic parameters for various kinetic models for Fenton process treating the biotreated neutral semi sulfite chemical (NSSC) effluent (Reaction time = 60 minutes) First-order

Second-order

Behnajady-Modirshahla-Ghanbery (BMG)

FeSO4.7H2O

H2O2

(mg/L)

(mg/L)

r2

k1

r2

k2

(min-1)

(L. mg-1. min-1)×1000

m

b

r2

(min)

50

450

0.0018

0.74

0.0008

0.85

60.2612

7.1818

0.93

50

3150

0.0035

0.58

0.0010

0.61

27.2010

6.6083

0.92

50

6300

0.0038

0.64

0.0011

0.67

27.6752

5.7460

0.98

500

450

0.0024

0.61

0.0007

0.62

55.5804

8.8173

0.94

500

3150

0.0055

0.83

0.0017

0.87

29.9547

4.1503

0.96

500

6300

0.0057

0.69

0.0017

0.75

24.3546

4.0736

0.90

1000

450

0.0026

0.65

0.0007

0.71

22.6154

8.4676

0.95

1000

3150

0.0099

0.91

0.0034

0.95

19.5849

2.5121

0.96

0.0074

0.94

0.0023

0.96

42.0774

2.6779

0.91

1000

6300

Table 4: The kinetic parameters for various kinetic models for Fenton process treating the biotreated RME effluent (Reaction time = 60 minutes) First-order

Second-order

Behnajady-Modirshahla-Ghanbery (BMG)

FeSO4.7H2O

H2O2

(mg/L)

(mg/L)

k1

r2

(min-1)

k2

r2

(L. mg-1. min-1)×1000

m

b

r2

(min)

50

450

0.0065

0.59

0.0172

0.67

7.94

3.86

0.99

50

3150

0.0088

0.81

0.0244

0.87

20.52

2.71

0.95

50

6300

0.0057

0.64

0.0150

0.71

12.29

4.24

0.99

500

450

0.0209

0.83

0.0805

0.93

6.59

1.52

0.99

500

3150

0.0154

0.79

0.0498

0.89

5.81

1.96

0.99

500

6300

0.0114

0.94

0.0355

0.97

19.24

2.24

0.87

1000

450

0.0233

0.78

0.0948

0.90

2.56

1.44

0.99

1000

3150

0.0110

0.81

0.0334

0.88

13.03

2.30

0.98

1000

6300

0.0098

0.97

0.0289

0.99

31.01

2.32

0.87

Table 5: Comparison between a one-step first-order, BMG and TSF kinetic model for the complete 240 minute reaction time (NSSC mill effluent) First-order

TSF model

BMG model

model r2

FeSO4.7H2O

H2O2

kapp,1

(mg/L)

(mg/L)

(min-1)

500

3150

0.0025

0.79

500

6300

0.0020

1000

3150

1000

6300

r2

m

b

r2

1.23E-03

0.99

78.12

2.5

0.95

0.13430

6.17E-04

0.99

42.87

3.20

0.97

0.57

0,04588

1.18E-04

0.99

26.076

2.25

0.97

0.74

0.06326

7.63E-04

0.99

50.98

2.68

0.98

y/x

k12

k13

(min-1)

(min-1)

0.79

0.07107

0.51

0.79

0.0032

0.40

0.0022

0.66